Method for generating steam



IN V EN TORS 2 Sheets-Sheet l v \CF ITICAI. SPECIFIC VOLUME FIG. I

D. N. FELGAR ETAL METHOD FOR GENERATING STEAM 900 1000 TEMPERATURE F 2L9 Q1. #UW

FIG 3 CRITICAL POINT Oct. 4, 1960 Filed Oct. 21, 1955 Donald N. FelgarBY Naihan L.Dickinson ATTORNEY Oct. 4, 1960 D. N. FELGAR ETAL 2,

METHOD FOR GENERATING STEAM Filed Oct. 21 1955 2 Sheets-Sheet 2 FIG. 2

CONSTANT CRITICAL SPECIFIC VOLUAQ/ aooc NORMAL D ERESSURE 50C URVE /?I-\F \E I M4000 n: PRESSURE LURVE E WITH TRANSITION IN 0) UNHEATED ZONE Ill,5 i CRITICAL I POINT sooc SATURATION CURVE 2000 FLUID TEMPERATURE FFIG. 4 24 26 INVENTOR-S Donald N. Felgar BY Narh an L.Dickinson ATTORNEYUnite IVIETHOD FOR GENERATING STEAM Filed Oct. 21, 1955, Ser. No.542,055

2 Claims. (Cl.122-4'59) The invention relates in general to a method andapparatus for generating vapor and, more particularly, is concerned witha method and apparatus for generating vapor while the vaporizable fluidis above the critical pressure.

A steam generator operating above the critical pressure of water (3206.2p.s.i.a. and 705.4 F.) produces steam from liquid water under conditionsquite unlike the generation of steam from the boiling phenomenon atpressures below the critical pressure. When steam is generated at apressure above the critical, there is a continuous rise in temperature,and the conversion from water to steam occurs without the presence ofvapor and liquid of different densities. Further, the fluid exhibitsgas-like properties at the critical pressure and above the criticaltemperature. Because of these properties, the prior art super-criticalvapor generators have been of the once-through type in which water ispumped up to pressure at or above the critical and flows through aheating zone in a confined flowpath, and issues from the end of thatpath as steam above the critical temperature of 705.4 F.

Once-through type boilers operating at pressures below the criticalpressure have involved diificulty during operation due to the depositionof the liquid dissolved solid impurities at a position where the finalliquid is evaporated. In that case, however, there are two phasessimultaneously existing and the liquid holds all the solids. Depositionoccurs when all of the liquid is evaporated.

Few steam generating units have operated at pressures above thecritical, and thus, there is little experience to indicate whether ornot the deposition of solid impurities in the liquid occurs duringtransition from water to steam. It has generally been assumed, based onthe kinetic theory of gases, that conversion to steam must take place ator about the critical temperature of 705 .4 F. but because of thenon-ideal nature of H there has been no clear evidence as to whether ornot deposition actually occurs during this conversion.

We have discovered that the deposition of the liquid carried solids doesoccur when supercritical pressure water is heated and, further, that thedeposition occurs at temperatures at or above the critical temperature,70.5.4 F. Moreover, we have discovered that in some circumstances asolid deposition occurs at fluid temperatures and pressures considerablyabove the critical. For instance, at 7500 p.s.i.a. deposition occurs atapproximately 880 F. We have also discovered that the deposition: occurswhen the fluid passes through a specific volume equal to that existingat its critical point of 3206.2 ns-.ia. and 705 .4 F. and that thiscritical specific volume occurs at temperatures above the criticaltemperatures as adirect function of pressure.

This discovery when applied to a steam generating unit operating withwater above its critical pressure is of considerable. practicableimportance because it promotes such design and operation of steamgenerating units that 2,954,758 Patented Oct. 4, 1960 they maysuccessfully cope with the detrimental eifects of deposition, such ashigh tube temperature and corrosion.

Accordingly, the present invention is directed to a method of generatingsteam above the critical pressure of 3206.2 p.s.i.a. in which the wateris pumped up to a predetermined pressure and then directed in a confinedstream through a heat transfer zone to receive heat from a sourcethereof. The heat absorption of the fluid is controlled so as tomaintain the point at which the fluid reaches its critical volume at apredetermined position in said heat transfer zone, said critical volumebeing determined from a known pressure-temperature-specific volumerelationship of the steam, and the critical specific volume beingdefined as a specific volume of the fluid at its operating pressureequal to the specific volume of water at its critical point (3206.2p.s.i.a. and 705.4 F.).

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this specification, but for a better understanding of theinvention, its operating advantages and specific objects attained by itsuse, reference should be had to the accompanying drawings anddescriptive matter in which there is illustrated and described apreferred embodiment of the invention.

In the drawings:

Fig. l is a curve showing the pressure-volume-temperature relationshipfor water above the critical pressure;

Fig. 2 is a curve showing pressure-temperature curve of the criticalconstant volume line;

Fig. 3 is a schematic diagram illustrating the operation of theinvention;

Fig. 4 is an alternative apparatus for carrying out the invention.

Fig. 1 graphically presents the relationships of the specific volume ofwater above the critical pressure under various conditions of pressureand temperature. It is intended that the term, water, is used to definethe molecule H 0 and the definition is not limited to the liquid phaseonly. The specific volume increases with temperature increases underconstant pressure heating conditions. Also indicated in Fig. l is thecritical point on the critical pressure line which defines the pressure,temperature andspecific volume at the point. These values (3206.2p.s.i.a., 705.4 R, 0.0503 cu. ft./lb.) are well established by a numberof experimenters and may be found in any of the usual referencematerials. Drawn through the critical point of this curve there is thedotted line denoting a constant critical volume line.

Fig. 2 is a cross plot of Fig. l in which pressure and temperature arethe co-ordinates and the constant critical specific volume line is drawnthereon. Further indicated is the line CDE which represents theprogressive states of the fluid passing through a supercritical pressuresteam generator and shows the temperature rise and the pressure drop. Atthe intersection of the line CDE with the critical specific volumecurve, we have discovered that depositions occur in the steam generator.Thus, it can be seen that deposition occurs not at the criticaltemperature exclusively but at various temperatures above the critical.For instance, at 7500 p.s.i.a. deposition occurs at 880 F. and isconsiderably above the critical temperature of 705.4 F. If a steamgenerator were arranged and operated on the assumption that thetransition or deposition occurs at 705.4" F. and at a particularposition in the steam generator, the unit would be found to be operatingincorrectly. The deposition would actually occur some 174 F. removedfrom this estimated position and might be at a position where theresultant corrosion and/or high tube temperature would make the unitunsafe.

In Fig. 3 the invention is indicated as applied to a once-through steamgenerator in which the feed pump 10 pumps water to a pressure greaterthan 3206 p.s.i.a. and causes it to flow through conduit 12. Thepressure in the conduit 12 is maintained by an outlet valve 14 and theconduit is divided into two heating zones, a liquid heating zone 15, anda steam heating zone 16. Temperature and pressure measuring devices 19,20 are placed at position 18 intermediate of the two heating sectionsand adjacent the outlet valve 14. Thus, as a fluid flows through theconduit 12 and is heated from a source (not shown) the temperature andpressure distribution of the unit is indicated by the pressure andtemperature instruments 19 and 20. With the relationships indicated inFigs. 1 and 2, the temperature at which the critical volume occurs maybe selected from the constant critical specific volume line for thepressure at the position 18. This temperature will indicate the positionwhere deposition occurs. By regulating the pump flow rate and the, heatinput rate into the'flowing fluid, the deposition or precipitated solidsmay be laid down in the conduit 12 in the position 18 and this positionis arranged in the unit so as to extend the life of the conduit whilemaking the heating process more efficient. The control of heat inputinto the water may be attained by any appropriate means, such as tiltingburners, gas recirculation, heated fluid recirculation and control ofthe heated fluid flow rate through the conduit to produce a proper heatcontent per pound of fluid flowing at the point of precipitation. Thesemeans control heat absorption of the flowing fluid in two basic ways;control of heat input to the tubes containing the fluid, and control ofthe fluid flow rate to vary the heat absorbed per pound of fluidflowing. Gas recirculation and tilting burners are Ways of controllingheat inputs while water circulation and flow rate control are ways ofcontrolling the heat absorbed per pound. All of these ways, however, canbe used to control the heat content of the water at any particularposition in the steam generating unit.

Knowing that, irrespective of the fluid-pressure, deposition alwaysoccurs when the fluid reaches a specific volume equal to the specificvolume at the critical point, the vapor generation process can bearranged to occur at any given position in a heat exchange zone. This isdone by measuring and controlling the pressure and temperature of thefluid so that the corresponding critical volume for that pressure occursat a predetermined position to thus have the deposition occur there.This is a method of generating vapor which may be applied to water underany pressure conditions above the critical so as to most effectivelygenerate vapor.

In Fig. 4 there is indicated an alternative method of generating vaporin which the fluid is pumped through a conduit 21 by a feed pump 22. Theconduit has a preheating portion 24 and superheating portion 26. Thesetwo portions are separated by a connecting line 28 having a valve 30therein and arranged with .a by-pass line 32 having a valve 34 therein.At the outlet end of the superheating section 26 there is an outletcontrol valve 36. Pairs of pressure and temperature measuring devices38, 39, and 40 are located in the bypass line 32, in the connecting line28 and just upstream from the outlet valve 36, respectively. In thisarrangement the water is pumped to a pressure above the critical by thepump 22 and is heated from a source (not shown) as itpasses through thepreheating section 24. Normally, the water flows through the open valve30 in the connecting line 528 and is superheated in the superheatingsection 26. Theme it passes to a point ofuse through the outlet con trolvalve 36. 111 the event that the feed liquid becomes contaminated bycondenser leakage, for example, the connecting valve 30 is closed andthe bypass valve 34 is opened to allow the preheated fluid to passthrough the bypass conduit 32. The heat input to the pressurized ifluidis regulated so that the critical constant .volume occurs atapproximately the position of the control valve 34. The regulation ofheat input based .on the temperature and pressure measurements beforethe control valve. The valve 34 is arranged to provide an essentiallyadiabatic throttling process in the fluid passing through the valve.Thus, pressure drop is controlled to be sufficient to cause the highpressure fluid to pass through its critical volume during the adiabaticthrottling process. The outlet valve36 maintains the steam pressureabove the critical. The control could be by hand or any well-knownautomatic means.

The throttlingprocess is indicated in Fig. 2 where the line ODErepresents the normal pressure and temperature distribution of the fluidpassing through a supercritical pressure steam generator. The line DFErepresents the fluid conditions as: it passes through the unit. Thethrottling process is represented by the line DF, thus with the criticalvolume occurring during a pressure expansion, the solids are depositedin the connecting line 32. The connecting line is arranged to beunheated so that the deposition would not interfere with the operationof the vapor generating unit.

This new method provides for the operation of a supercritical pressuresteam generator for a considerable period of time without having to beshut down due to detrimental effects on the tubes from solid deposition.

The sources of thermal energy have not been shown or described becausethe invention may be carried out with any of the well-known types suchas, hot liquid or hot combustion products.

Further, the process is considered a method for separating liquidcarried solids from water above its critical pressure. Under suchcircumstances, methods of mechanical separation are inadequate becausethe fluid never contains two phases, i.e. steam and water, each of adifferent density.

While in accordance with the provisions of the statutes, we haveillustrated and described herein a specific form of the invention nowknown to us, those skilled in the art will understand that changes maybe made in the form of the apparatus disclosed without departing fromthe spirit of the invention covered by our claims, and thatcertaincfieatures of the invention may sometimes be used to advantagewithout a. corresponding use. of the other features.

We claim:

1. The method of generating steam in a forced circulation once-throughvapor generator in the range of pressures above the critical pressure of3206.2 p.s.-i.a. comprising supplying solids-bearing Water at a pressuregreater than said critical pressure and ate. temperature less than thecritical temperature of 705.4" F., conduct ing the pressurized water inheat transfer relationship with a thermal energy source so as toincrease its temperature above 705.4" F., and controlling the positionwith respect toithe thermal energy source at which the fluid reaches itscritical specific volume within said range of pressures and therebyprecipitates the liquid-home solids by regulating the heat absorbed bythe fluid, said critical specific volume for said range of pressuresbeing the specific volume corresponding to the specific volume whichwould exist were the water at its critical point, 3206.2 p.s.i.a. and705.4 F., the temperature at which said critical specific volume occursbeing above 705.4 and in the uppermost portion of said range ofpressures being at least degrees F. above the temperature of 705.4" F.

2. The method of generating steam in a forced circulation once-throughvapor generator in the range of pressures above the critical pressure of3206.2 p.s.i.a..

comprising supplying solids-bearing water at a pressure greater thansaid critical pressure and at a temperature less than the criticaltemperature of 705.4 F., conducting .the pressurized water in heattransfer relationship with a thermal energy sourceso as to increase itstemperature above 705.4"v F. to a point where the heated fluid is in astate which is just before itreaches its crit cal specific volume, saidcritical specific volume for said range of pressures being a specificvolume corresponding to the specific volume which would exist were theWater at its :critical point, 3206.2 p.s.-i.a. and 705.4 F., thetemperature at which said critical specific volume occurs being above705.4 F. in the uppermost portion of said range of pressures being atleast 100 degrees F. above the temperature of 705.4 E, and adiabaticallyexpanding the heated fluid through a pressure range above the criticalpressure to cause said fluid to pass through its critical specificvolume at a predetermined position out of contaot with said thermalenergy source to precipitate said water borne solids.

References Cited in the file of this patent UNITED STATES PATENTS2,074,235 Muller Mar. 16, 1937 5 2,211,724 Kerr Aug. 13, 1940 FOREIGNPATENTS 400,164 Great Britain Jan. 15, 1932 443,771 Great Britain Mar.5, 1936 400,163 Great Britain 1933 OTHER REFERENCES Steam PowerEngineering by MacNaughton (3rd Ed.) of 1948, pages 60 to 65.

